Products powered by electrochemical devices are limited by the useful life of the electrochemical device (i.e., the ability of the electrochemical device to supply energy to the application). For example, the useful life of a flashlight is limited by the ability of a cell to provide sufficient current to the flashlight lamp or light-emitting diode. This useful life is dependent on several factors not controlled by the user such as rate of discharge, age of the electrochemical device, and environment where the device is used. The time at which an electrochemical device reaches the end of its useful life may be termed the “End of Life” (EOL) of the device.
In applications powered by electrochemical devices comprising a primary cell, knowledge of an EOL condition of the cell may alert the user that it is time to replace the cell. In applications powered by electrochemical devices comprising a secondary cell, the EOL condition may signal to the user that it is time to charge the cell. For example, if a flashlight user is made aware that the EOL condition of the cell is approaching, the cell may be replaced or charged when the flashlight is not in use so that the flashlight may provide light during all times of use.
Lithium electrochemical cells, which are more commonly referred to as batteries, are widely used in a variety of military and consumer products. Many of these products utilize high energy and high power batteries. Due in part to the miniaturization of portable electronic devices, it is desirable to develop even smaller lithium batteries with an increased power capability and service life. One way to develop smaller batteries with increased service life is to develop higher energy cathode materials.
One example of a high energy cathode material is fluorinated carbon (i.e., CFx). CFx is often used with a lithium anode in non-rechargeable (primary) batteries for, among other things, military devices and implantable medical devices. CFx (where x=1.0) has a specific energy of about 860 mAh/g. Other examples of high energy cathode materials include silver vanadium oxide and manganese dioxide, which have specific capacities of about 315 and 308 mAh/g, respectively.
The cathodes for rechargeable (secondary) batteries, such as Li ion batteries, generally have lower energy storage capability than primary battery cathodes. However, secondary batteries can typically be recharged several hundred times, which significantly reduces the lifetime cost as well as the battery disposal costs. Examples of secondary battery cathodes used in Li ion batteries include lithium cobalt oxide, lithium iron phosphate, and lithium nickel cobalt oxide.
To satisfy the demands for longer lasting or smaller batteries, there continues to be a need to develop cathodes exhibiting higher energy like primary batteries with the possibility of partial or fully rechargeable capability like secondary batteries, thus extending lifetime and effectively reducing the overall cost. Mixed cathode materials have been proposed as one possible approach for achieving such improved primary and/or secondary batteries. Other benefits of mixed cathode materials include enhancing the rate capability and/or stability of the cathode, while maintaining the energy density per weight and/or per volume. Approaches for achieving such benefits have typically involved mixing a high rate-capable cathode material with a high energy-density cathode material.
U.S. Pat. No. 7,476,467 discloses a cathode material for secondary lithium batteries. The cathode active material comprises a mixture of (A) a lithium manganese-metal composite oxide having a spinel structure, and (B) a lithium nickel-manganese-cobalt composite oxide having a layered structure. The cathode active material is said to have superior safety and a long-term service life at both room temperature and high temperature due to improved properties of lithium and the metal oxide.
During discharge, some cells (e.g., CFx based cells) exhibit an interval where the discharge voltage of the cell remains relatively constant. Near the EOL of the cell, the discharge voltage decreases rapidly with respect to discharge capacity and/or time. That is, a voltage discharge curve that is a plot of discharge voltage versus specific capacity or time displays a large negative slope. A decrease in voltage below the relatively constant discharge voltage may not provide useful information about the EOL condition because of the coinciding rapid drop in discharge voltage. In other words, very little time exists between the beginning of the sharp voltage drop and the actual EOL of the cell. Additionally, the slope of the voltage discharge curve varies from cell to cell because of variances that exist and/or are inherent in cell materials and cell manufacturing processes. The large negative slope of the voltage discharge curve and its variability from cell to cell affects the accuracy, repeatability, and utility of using voltage measurements at the terminals of the electrochemical device to determine the EOL condition of a cell.
In critical applications or applications where the cell is not readily accessible (i.e., the cell may not be easily replaced), information regarding the EOL condition may be transmitted to a user of a device, or to another party. Examples of these types of applications are implantable medical devices, devices used for remote sensing of earthquakes, volcanoes, tsunamis, or other environmental conditions, or devices for military/law enforcement communications during training and combat missions. In each of these examples, the cell may not be readily accessible, and failure to perform required functions due to a low battery condition may not be an acceptable outcome.
It is known to those skilled in the art that composite cathodes comprising fluorinated carbon with some other metal oxide are used for the purpose of providing the battery with an end-of-life (EOL) indicator (i.e., providing a short interval of useful discharge voltage after the interval of relatively constant discharge voltage). For example, U.S. Pat. No. 5,667,916 describes a battery having a cathode mixture of CFx and other materials, including for example copper oxide, the other material or mixtures of other materials serving as the end-of-life indicator. Similarly, U.S. Pat. No. 5,180,642 discloses electrochemical cells or batteries having a cathode mixture comprised of manganese dioxide (MnO2), carbon monofluoride (CFx, where x=1), or mixtures of the two, and an end-of-life additive selected from the group consisting of vanadium oxide, silver vanadate, bismuth fluoride and titanium sulfide. U.S. Pat. No. 4,259,415 provides a cathode material as an end-of-life indicator comprising a main positive active material and a precursor. Suitable main positive active materials include molybdenum oxide (MoO3), silver oxide (Ag2O), and graphite fluoride (CF)n.
Although many batteries or cells developed to-date include end-of-life indicators, the energy density is less than desired. The capacity (e.g. mAh/gm or mAh/cc) of the EOL additive to CFx (for example, silver vanadium oxide, or SVO) is lower than that of the CFx material, resulting in a composite electrode with a total capacity lower than that of the CFx by itself. Additionally, or alternatively, many electrochemical devices, batteries, or cells developed to-date exhibit an initial voltage sag or drop at the beginning of the discharge. Therefore, a need continues to exist for improved cells, and more particularly for improved cathode materials for use in such cells, detection of an EOL condition in such cells, and transmission of the EOL condition to a user of the cell.